Conventional radiation-emitting semiconductor components often have a rectangular shape for reasons of production technology. The semiconductor components generally comprise a multilayer structure with an active, radiation-generating layer, said multilayer structure being deposited epitaxially on a carrier substrate. The carrier substrate is preferably electrically conductive in order to enable a vertical current flow. Moreover, it is expedient in many cases if the carrier substrate is transparent to the radiation generated in the active layer of the multi-layer structure. However, a high transparency is often at odds with a high electrical conductivity of the material for the carrier substrate. Thus, by way of example, sapphire used for GaN-based light-emitting diodes is transparent to blue light but is not electrically conductive. By contrast, although silicon carbide as carrier substrate for GaN light-emitting diodes is conductive and transparent, the transparency decreases as the conductivity increases, with the result that the properties of the semiconductor component are not ideal in this case either.
Therefore, one possibility for reducing the absorption losses and thus for increasing the external efficiency is the removal of the carrier substrate and to apply suitable mirror layers to the component (thin-film concept). However, a semiconductor thin film is essentially a co-planar plate whose coupling-out efficiency is not increased compared with a standard diode on account of the geometry. Particularly if a carrier substrate exhibiting only little absorption (for example GaN on SiC) has already been used for the semiconductor component, the increase in the external efficiency of the thin-film semiconductor component is too small to justify the increased technical effort for removing the carrier substrate.
In order to elucidate the problem area of coupling out radiation, FIG. 8 schematically shows a semiconductor component with the cones of coupling out radiation. Radiation can be coupled out of the semiconductor component only from a cone with an aperture angle of θ=sin−1 (πext/πint), where πint denotes the refractive index of the semiconductor material and πext denotes the refractive index of the surroundings. For a GaN semiconductor (πint=2.5), the coupling-out angle θ is 23° with respect to air (πext=1) and 37° with respect to a plastic encapsulation (πext=1.5). Radiation that is generated in the semiconductor component and does not impinge on the interfaces within a cone is finally reabsorbed and converted into heat. Although the coupling-out cone is large for GaN systems in comparison with GaAs systems (πint=3.5), it nevertheless leads to undesirably high radiation losses.
These conditions also do not change significantly with altered layer thicknesses. However, the thin-film geometry is expedient for the beam coupled out via the top side since the absorption is low on account of the short path in the semiconductor; for the beam coupled out laterally, by contrast, the efficiency may even be lower on account of the multiple reflections in the semiconductor.
Therefore, there are already various approaches for increasing the external efficiency of semiconductor components through altered geometries. Mention shall be made here, in particular, of a so-called micropatterning of the entire multilayer structure, which leads to an intensified lateral coupling out of radiation on account of the larger total area of the side areas of the multilayer structure. In addition, the side areas of the individual multilayer structures thus produced may be beveled. Examples of such semi-conductor components are disclosed in DE-A-198 07 758, (corresponding to U.S. Pat. No. 6,229,160) EP-A-0 905 797 (corresponding to U.S. Pat. No. 6,111,272) or JP-A-08-288543.
A further possibility for increasing the coupling out of radiation is shown in FIGS. 3 and 5 of DE-A-199 11 717. Here, the multilayer structure with the active, radiation-generating layer is assigned individual radiation coupling-out elements in the form of sphere segments or truncated cones formed for example by means of corresponding etching of grown layers.
However, none of the documents cited with respect to the prior art deals with GaN-based thin-film semi-conductor components. GaN-based semiconductor components predominantly serve for generating radiation in the blue-green spectral range and have a plurality of layers comprising a GaN-based material. In the context of this invention, a GaN-based material is understood to mean not only GaN itself but also materials derived from GaN or related to GaN and also ternary or quaternary mixed crystals based thereon. What are included in particular in this respect are the materials GaN, AlN, InN, Al1-xGaxN, In1-xGaxN, In1-xAlxN and Al1-x-yInxGayN where 0<x<1, 0<y<1 and x+y≦1.